There are many instances when analog output signals are developed by simple RC integrating circuits from pulses having durations representative of digital inputs. The aforementioned copending application, which provides one example, is directed to a non-contacting sensor apparatus in which the position of a moving part of the sensor is represented by the relative time durations of two output pulses of a counter. The two pulses are developed by first counting a prescribed number of pulses of a first series of input pulses representative of the resonance frequency of a first tank circuit which, in turn, represents the position of the moving part of the sensor relative to a first stationary inductance coil and then counting the same number of pulses of a second series of input pulses representative of the resonance frequency of a second tank circuit which, in turn, represents the position of the moving part of the sensor relative to a second stationary inductance coil. The relative times required to count the prescribed number of pulses define the time durations of the counter output pulses. An RC integrating circuit develops an analog output signal, representative of the position of the moving part of the sensor, from the counter output pulses.
The output signal of an RC integrating circuit is composed of a series of rising and decaying portions. In certain applications, such as when the time constant of the RC integrating circuit is relatively small, changes in an analog output signal are too large relative to the time over which the changes occur and produce undesirable or even unacceptable results. Meters and other display devices, arranged to faithfully indicate the average value of the parameter being measured or monitored, cannot respond to such changes in the signal produced by an RC integrating circuit to provide an accurate reading.
Consequently, it is preferable or even necessary to develop an analog output signal which represents or closely approximates the average of the signal developed by the RC integrating circuit.
One solution to the problem of large changes in the analog output signal is to increase the time constant of the RC integrating circuit to such an extent that the changes in the analog output signal are small and deviate little from the desired average value of this signal. However, large time constants slow down the response of the RC integrating circuit, so that quick changes in the parameter being displayed will not be indicated if they are too fast relative to the response time of the RC integrating circuit.
Faced with the requirement of small time constants for RC integrating circuits having quick response times, circuits have been developed in the past which selectively sample the analog output signal of an RC integrating circuit. By developing a signal from selected parts of the changing signal developed by the RC integrating circuit, the changes in the analog output signal are relatively small because the analog output signal is derived from limited changes in the signal developed by the RC integrating circuit.
Although such a sampling technique provides an improvement, the analog output signal still contains undesirable changes because the sampled signal is changing during sampling. Such changes in the analog output signal still introduce errors in the indication of the parameter being measured or monitored.